Porous electrode-support for alkaline accumulators

ABSTRACT

A porous electrode support for alkaline accumulators consisting of a sintered mass of chemically nickel-coated graphite particles with a particle size of 5 to 200 Mu . A reinforcing fiber layer is sintered to the particles and can consist of synthetic resin or vitreous fibers coated with nickel. Preferably the fiber reinforcing mats or webs are provided on both faces of the flat support.

llite ties atent Faber 1151 3,67,014 1451 Apr. 10, 1972 [54] POROUSELECTRODE-SUPPORT FOR ALKALINE ACCUMULATORS [72] Inventor:

[73] Assignee: Rheinisch-Westfalisches-Elektrizitatswerk,

Essen, Germany 1 l t rrlfabsrs i blc- Meiafismenx [22] Filed: Oct. 19,1970 [21] Appl. No.: 82,089

Related US. Application Data [63] Continuation of Ser. No. 834,683, June19, 1969.

[30] Foreign Application Priority Data June 20, 1968 Germany ..P 17 71646.0

[52] US. Cl ..136/64, 29/1825 [51] Int Cl ..H01m 35/04 [58] Field 61Search ..136/28, 29, 64, 65, 120.75, 136/121-122, 57, 74; 75/200, 201,202, 212, 222, 206; 29/1825 [56] References Cited UNITED STATES PATENTS747,454 12/1903 Lowendahl ..75/201 1/1960 Great Britain ..29/182.5

Primary Examiner-Anthony Skapars Attorney-Karl F. Ross [57] ABSTRACT,

A porous electrode support for alkaline accumulators consisting of asintered mass of chemically nickel-coated graphite particles with aparticle size of 5 to 200 71.. A reinforcing fiber layer is sintered tothe particles and can consist of synthetic resin or vitreous fiberscoated with nickel. Preferably the fiber reinforcing mats or webs areprovided on both faces of the flat support.

4 Claims, 3 Drawing Figures PATENTEDAFR 18 1912 Fig] r e w F r e a P Inventor.

POROUS ELECTRODE-SUPPORT FOR ALKALINE ACCUMULATORS This application is acontinuation of my copending application Ser. No. 834,683, filed June19, 1969.

The present invention relates to a porous electrode support for alkalineaccumulators and, more particularly, to a support for use innickel-zinc, nickel-cadmium or like rechargeable electrochemicalsystems.

It is a common practice in connection with alkaline accumulators, i.e.rechargeable electrochemical systems, especially batteries using analkaline electrolyte, to provide the active material of one or both ofthe electrodes upon a porous or electrolyte-permeable support. Thissupport, which is electrically conductive, serves as a current-supply orcurrent-collection network in intimate contact with the active materialof the electrode. Such supports have been provided in many forms, forexample as screens or grids made of metal wire, metallized fiber and thelike, stampedor punched-metal frameworks, pocketed metal plates,sintered-powder bodies in which the active material is disposed withinthe pores and on the surfaces of the support, and the like.

In other electrode structures, the active material is retained in apackage or envelope and merely is a current-collecting orcurrent-distribution network.

In most cases, however, the support must be capable of carrying theactive material without appreciable change of shape of the electrode,must be capable of withstanding mechanical, chemical and electricalstresses arising in the electrodepack, and must be stable against anydeterioration in the electrolytic medium in which the cell is operated.

Mechanical stresses, for example, may arise from swelling of theelectrode pack which may have separators fitted tightly betweenelectrodes of opposite polarity, or from the generation of gas onelectrical charging or discharging of the cell. Moreover, high rates ofcharge and discharge of the cell give rise to the evolution of heatcausing thermal stresses which tend to expand or contract the electrodeand, in an unstable system, may cause slouging of the active electrodematerial or other deterioration of the cell structure.

Finally, an important consideration in the choice of construction of asupport for the active material in a battery or other rechargeablesystem, is the uniformity of the current distribution and currentcollection to avoid concentration of current flow at limited regions ofthe plate. On charge, especially when the electrode is used inconjunction with a replatable electrochemical mass, such chargeconcentrations or localized elevated current densities may result inlocalized concentrations of the active material with a variation inplate reliability, ampere-hour capacity, etc.

It has been proposed earlier to provide a support for the activematerial of an electrode, especially for nickel alkaline accumulators orstorage cells in which nickel-plated graphite particles are sinteredtogether, in conjunction with finely divided nickel dust. In suchsystems, the graphite particles have a particle size which is relativelylarge, i.e. well over 200 p. in particle diameter.

The graphite particles are galvanically plated with nickel, i.e.electroplated, and thereafter sintered into a plate-like body. Suchsystems have, however, the disadvantage that a galvanic nickel platingupon graphite particles cannot be held uniform and, when these particlesare combined with nickel dust in an electrode support of the characterdescribed, they consume more nickel than is necessary.

Furthermore, in mechanical respects, electrode supports of the characterdescribed are also not satisfactory. Firstly, the support is brittle andeasily broken upon subjection to mechanical stress. Secondly thenonuniformity of the electroplating of nickel upon the graphiteparticles, which, as mentioned earlier, have a particle size far inexcess of 20011., results in a defective sintering of the support into acoherent body. More specifically, regions at which insufficient nickelis present as a coating of the graphite particles fail to sinter at all,while regions in which excessive nickel is present sinter to a greaterextend than other regions of the coating. Consequently, portions of theelectrode support are mechanically unstable. Lastly, in this connection,it may be noted that a nonuniform coating of the graphite particles,which are to be rendered coherent as part of the support for theelectrochemically active material of an alkaline accumulator or storagecell, gives rise to nonuniforrnity of current distribution on charge anddischarge cycling with the disadvantages with respect to current densityenumerated earlier.

While it has been proposed to coat sheet-or foil-like substrates ofsynthetic resins or fibers of various types with nickel in nongalvanicsystems, eg nonelectrical plating, it has not in general been possibleheretofore to provide such coatings on graphite particles of the typeincorporated in electrodes as mentioned earlier. Such nonelectricalcoating or so-called chemical plating of nickel makes use of aprecipitation, generally in heated solution and preferably at or closeto the boiling point thereof, of metallic nickel by reducing nickel ionsavailable in the solution. In synthetic-resin coating by these methods,considerable preparation of surface to receive the nickel coating isrequired. Metals which are electrochemically remote from the noblemetals in the electrochemical series, such as iron or copper, acceptnickel directly from solution. Compact or bulk graphite, however,appears to be absolutely immune to chemical plating with nickel by anyof the known processes of the general type described above. Even surfacetreatments, short of actually coating the graphite with other materials,fails to render the nickel acceptable by the graphite.

It is, therefore, the principal object of the present invention toprovide a porous electrode support for an alkaline accumulator wherebythe aforementioned disadvantages are obviated, and a mechanically stableelectrode is constituted.

Another object of this invention is to provide an improved nickelsupport grid or porous electrode structure having better mechanical andelectrical properties by comparison with earlier support structures.

Still another object of the present invention is to provide aporous-powder electrode support, especially for nickel alkalineaccumulators, having high structural strength, ability to withstandrepeated charge-discharge cycling, resistance to mechanical and thermalstress, and the uniformity and unalterability requisite in such systems.

I have now found, most surprisingly, in spite of the fact that compactgraphite is almost completely immune to coating by chemical plating ofnickel onto substrates under high-temperature processing, that it ispossible to chemically plate graphite particles of a particle size of 5to 200p by conventional nonelectrical or electroless plating techniquesat or close to the boiling point of the chemical-plating solution.

Consequently, the present invention provides, according to an essentialfeature thereof, that the electrode structure is composed at least inpart of a sintered mass of graphite particles having a particle size of5 to 200p., chemically plated with nickel. The nickel-coated graphiteparticles, which are found to have a highly uniform and continuoussheath of nickel, can be sintered together uniformly and economicallywith a minimum of diificulty to yield a highly stable electrodestructure resistant to mechanical, thermal and other stresses arising inalkaline-accumulator systems.

According to a further feature of this invention, porous sinterednickel-coated graphite particles are provided with at least one layer ofporous reinforcement in the form of a fibrous material which, moreover,is sintered to the nickelcoated particle body. While this layer may bereceived within the sintered-particle body, preferably it is applied atleast on one surface thereof and has at least a metal surface sinterableto the particles. Still more favorable is an arrangement in which such alayer is provided on each face of the sinteredparticle plate.

The reinforcement layer, according to this invention, is a mat, fleeceor fabric composed of nickel-coated fibers, preferably of syntheticresin or a refractory mineral material. The nickel-coated mat, fleece orfabric is preferably composed of glass fibers, synthetic-resinmonofilament (e.g. nylon) or natural fibers (e.g. of cotton) or thelike.

After nickel coating of the graphite particles of the limited particlesize mentioned earlier, these particles are sandwiched between twolayers of the reinforcement and sintered into a coherent body at atemperature of about 900 C.

In my application Ser. No. 668,006 filed 15 Sept. 1967, (now U.S. Pat.No. 3,476,604) I have described a grid for an electrode of anelectrochemical cell, e.g. a fuel cell or a unit of a primary orsecondary battery. Such grids serve the dual purpose of supporting theactive electrode material and providing a low-resistance current pathbetween the particles of such material. In that system, a web ofcarbonized and at least superficially graphitized filaments constitutesa coherent skeleton on which a metallic coating is deposited to conferthe necessary rigidity and conductivity to the web. The metallic coatingis preferably deposited by so-called electroless chemical platingwhereby the metal is precipitated from a solution of a compound in whichthe metal is the cation with the aid of a reducing agent. In the systemof that application, the coating consisted predominantly of nickel and asmall amount (about percent by weight) of boron and was deposited, forexample, from an aqueous solution of a nickel salt (e.g. nickel chlorideor nickel nitrate) in the presence of a reducing boron compound e.g. aborohydride such as lithium borohydride). The nickel coating of thatsystem was applied to a thickness of 5 to l0 .t.

In my application Ser. No. 504,263, filed 23 Oct. 1965, now U.S. Pat.No. 3,436,267, and copending with the first-mentioned application, Ihave described a method of making nickel compounds which may be appliedto such electrode grids for use in alkaline accumulators. Basically, thesystem makes use of a treatment of dry, divalent nickel hydroxide withan ozone-containing gas at a temperature between substantially 20 C. and1 C. for a period sufiicient to convert substantially all of the nickelhydroxide into a black mass of a higher oxidation state with an apparentformula of NiQOI-I. The divalent nickel hydroxide is agitated in the gasstream containing ozone and the latter may be formed in an oxygen gas byultraviolet irradiation. The treatment may be carried out in a gasvortex. This resultant compound may be used as the active material of anelectrode plate having the support described above in accordance withthe present invention. The fibers applied as reinforcements on eitherside of the sintered-particle mass may be those described in applicationSer. No. 668,006 or Ser. 624,646 filed 1 Mar. 1967. In the latterapplication, I have described arrangements wherein the fibers areelectrochemically coated in accordance with the principles ofU.S. Pat.No. 3,185,591 or U.S. Pat. No. 3,006,821.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. I is a diagrammatic cross-section through an electrode supportaccording to the present invention;

FIG. 2 is an enlarged detail view of a portion of the electrode-supportstructure of FIG. 1; and

FIG. 3 is a diagrammatic view of yet another electrode structure.

In FIGS. 1 and 2 of the drawing, I have shown an electrodesupportstructure 1 for an alkaline accumulator which comprises, basically, arelatively thick flat body or plate composed of graphite particles,chemically plated with nickel, sintered together at a temperature ofabout 900 C. As can be seen from FIG. 2, the graphite particles 2, whichhave a particle size of about 20p. but may range in particle sizebetween 5 and 20011 according to the principles of this invention, arecoated unifomily with a chemically deposited nickel layer 3 and aresintered at contact points between these coatings into a body with highmechanical strength and resistance to the stresses arising in aelectrode system. The sintered-powder body is sandwiched between a pairof nickel-coated fiber mats, which matylbe of a nonwoven pe,and/orfabrie webs represented at 4, ese reinforcements erng provided onboth outer sides of the powder mass. In FIG. 3 the reinforcement isshown to be a fabric web 24 received within the mass 22. The nickelcoatings of the fiber are sintered to the particles.

While any of the chemical-plating or electroless processes for the metalcoating of graphite particles may be used, I have noted in applicationSer. No. 668,006 that there is an advantage in including a small amountof boron in the nickel coating. The electroless process which is usedaccording to the present invention to coat particles of graphite havinga particle size 5 to 200p, but which is not capable of effectivelycoating bulk graphite, may be that described in U.S. Pat. No. 3,338,726.

As shown in FIG. 2, the particles 2 are not substantially rounded andmay have angular edges and generally flat faces which, nevertheless, areuniformly coated with the constant thickness of the nickel plate. Thisangular-edged configuration of the graphite particles, which may beobtained by milling, crushing or otherwise comminuting graphite,provides the advantage that a interparticle contact within the mass iseffected between angular junctions and flat surfaces. For example, inFIG. 2 angular edges at 3 and 3 are joined to flat surfaces at 3a and3a. At other junctions, e.g. at the junction 3b, angular edges of twoparticles are joined. With lesser frequency, flat surfaces of theparticles may be sintered together as shown at 3c.

EXAMPLE Using the bath and conditions set forth in U.S. Pat. No.3,338,726, a mass of graphite particles with a particle size of about 20p. is chemically plated with nickel to a uniform coating thickness ofabout Su. The mass is rinsed, pressed into sheets, sandwiched betweennickel-coated glass-fiber mats and sintered at a temperature of about900 C. The resulting plates may then be cut up for use in anelectrochemical cell and may receive a coating of the active material ofU.S. Pat. No. 3,436,267.

Iclaim:

1. A porous-powder electrode support for alkaline accumulators,comprising a porous body of coherent graphite powder particles in a sizeranging between substantially 5 and 200 11., said particles being eachcompletely ensheathed by a continuous nickel coating of substantiallyuniform thickness and being in sintered coating-to-coating contact withone another, and a reinforcement for said body comprising nickel-coatedfilamentary fibers in sintered coating-to-coating contact with oneanother and with said particles.

2. An electrode support as defined in claim 1 wherein said fibersconsist of synthetic-resin monofilaments.

3. An electrode support as defined in claim 1 wherein said fibersconsist of glass.

4. An electrode support as defined in claim 1 wherein said body is flatand said fibers form a layer on at least one major surface thereof.

2. An electrode support as defined in claim 1 wherein said fibersconsist of synthetic-resin monofilaments.
 3. An electrode support asdefined in claim 1 wherein said fibers consist of glass.
 4. An electrodesupport as defined in claim 1 wherein said body is flat and said fibersform a layer on at least one major surface thereof.